1. Field of the Invention
The present invention relates to a laser-based measuring apparatus and a method for measuring the length or distance of an object under measurement.
2. Description of the Related Art
An interferometer has been applied in measuring technology. The interferometer divides light from a laser light source into at least two coherent light beams, passes the divided light beams through different optical paths from each other, and makes the resultant light beams optically interfere with each other after recombination.
Measurements of length utilizing light waves are classified into a coincidence method which observes interference fringes at both ends of an object under measurement to measure the length of the object, and a counting method which employs an interferometer having a movable measuring reflector which is moved from a start point to an end point of a length under measurement to count bright and dark regions in interference fringes generated therebetween. The counting method is implemented in a laser-based measuring apparatus using a laser light source, which is widely employed for precise length measurements.
FIG. 1 illustrates the configuration of a laser-based measuring apparatus (linear interferometer) for the most basic two-wavelength type moving interferometer. A He—Ne laser,which is a laser light source, applies a magnetic field to a discharger to emit coherent light having two frequency components f1, f2, which are slightly different in frequency, by the action of Zeeman effect. The light beam components have planes of polarization orthogonal to each other such that two circularly polarized light beams are emitted in rotating directions opposite to each other. The two frequency components f1, f2 are both stabilized. The light beams are separated into two in the laser light source, and one of the light beams is opto-electrically transduced directly by a photodetector within the light source to output a beat signal at f1−f2 as an electric reference signal, while the other light beam having the components f1, f2 is output from the light source and enters an interferometer.
The light beam entering the interferometer is separated into two having the respective frequency components by a polarization beam splitter. One of the light beams, f1 is emitted to a measuring reflector attached to a moving object, such as a corner cube prism, for example, and reflected thereby to become measuring light. The other light beam f2 is reflected by a fixed reference reflector to become reference light. The resulting light beams are combined again by a polarization beam splitter, i.e., interfere with each other. A relative movement between the beam splitter and the measuring reflector causes the frequency of the measuring light to change by Δf due to the Doppler effect. In other words, with addition of a Doppler component, the light component f1 changes to f1±Δf. Light beams, which interfere with each other in the beam splitter, are opto-electrically transduced by a photodetector, and a signal to be measured f1−(f2±Δf) of a biased beat signal is generated as the difference of optical frequency by heterodyne detection. A measuring circuit only calculates ±Δf which is the difference between the signal to be measured f1−f2±Δf and the reference signal f1−f2 of the laser light source, and is converted to position information. Specifically, a frequency counter in the measuring circuit calculates a difference in count between the signal to be measured and the reference signal. This difference is multiplied by one half of the wavelength of the light beam to derive a moving distance of the measuring reflector with respect to the beam splitter.
Further, a technique for increasing the resolution of a laser measuring apparatus is implemented in a two-pass interferometer and a four-pass interferometer which pass the measuring light through an optical path between a beam splitter and a measuring reflector a plurality of times to increase a Doppler component for increasing the resolution.
FIG. 2 illustrates the configuration of a two-pass interferometer which optically improves the resolution of a laser-based measuring apparatus.
The two-pass interferometer employs a polarization beam splitter; a corner cube prism and a reference plane mirror opposing each other with the polarization beam splitter and optical axis interposed therebetween; and two quarter wavelength plates positioned on the optical axis and between the polarization beam splitter and corner cube prism. The plane mirror is used as a measuring reflector.
Two light beams at frequencies f1, f2 having orthogonal planes of polarization, emitted from a laser light source, are separated by the polarization beam splitter. One of the light beams having the component f1 is reflected by the polarization beam splitter and bent by 90 degrees, passes through the quarter wavelength plate, is reflected back by the fixed reference plane mirror, and again passes through the quarter wavelength plate. Therefore, since the plane of polarization of the reference light rotates by 90 degrees, the reference light again travels via the polarization beam splitter to the corner cube prism. The reference light folded back by the corner cube prism goes to the beam splitter, and is reflected again by the reference plane mirror. In this event, since the reference light passes through the quarter wavelength plate twice, the reference light is reflected and bent by the beam splitter at the second time, and returns toward the light source and impinges on a photodetector.
The other light beam having the component f2 goes to the beam splitter from the laser light source, and is reflected by the plane mirror functioning, and returns to the beam splitter as the measuring reflector. At this time, since the measuring light has passed through the quarter wavelength plate twice, the measuring light is reflected and bent by the beam splitter at the second time to reach the corner cube prism, folded back by the corner cube prism, and bent by the beam splitter, so that the measuring light again directs to the plane mirror which functions as the measuring reflector. The measuring light reflected by the plane mirror functioning as the measuring reflector and returning to the beam splitter, travels via the beam splitter, and returns to the light source, and impinges on the photodetector at this time, since its plane of polarization has rotated.
Therefore, the measuring light reciprocates twice between the interferometer and the measuring reflector to generate the Doppler component ±2Δf, so that the resolution is increased twice as high as that of the linear interferometer.
The four-pass interferometer has increased lengths of optical paths for reference light and measuring light, twice as long as those of the two-pass interferometer, so that the four-pass interferometer comprises one each of additional beam bender and corner cube prism for correspondingly increasing the lengths of optical paths for the reference light and measuring light. Since the measuring light reciprocates four times between the interferometer and measuring reflector, the measurement resolution is increased four times as high as that of the linear interferometer.
The two-pass interferometer and four-pass interferometer, which are techniques for increasing the resolution of laser measuring apparatuses, is provided with optical paths for an increased number of optics, and is configured to direct two or four light beams perpendicularly to a measuring plane mirror, a plane mirror having a relatively large diameter must be attached to an object under measurement. Therefore, these interferometers cannot be used when a small measuring reflective plane is measured due to a limited space, or when a measuring reflective plane is not plane, such as cylindrical, spherical and the like.